1) Field of the Invention
The present invention relates to a belt device that detects an actual belt speed and corrects a belt speed based on the actual belt speed, an image forming apparatus including the belt device, and a method to control belt speed.
2) Description of the Related Art
Image forming apparatuses such as copying machines and printers capable of forming a full color image are increasing with the demands of the market. Such an image forming apparatus includes a so-called tandem type image forming apparatus. This type of image forming apparatus includes a plurality of photosensitive elements that are arranged in tandem, and a plurality of developing devices that develop toners of different colors corresponding to the photosensitive elements. In this image forming apparatus, toner images each having a single color are formed on the photosensitive elements, and the toner images of the single colors are sequentially transferred to a belt-shaped or a drum-shaped intermediate transfer element to form a full-color composite image.
The tandem type image forming apparatus may include a direct transfer system and an indirect transfer system. In the image forming apparatus with the direct transfer system as shown in FIG. 12, toner images formed on photosensitive elements 91Y, 91M, 91C, and 91K aligned in a row are sequentially transferred, by transfer devices 92, to a sheet of paper P (hereinafter, “sheet P”) carried on a sheet conveying belt 93 that rotates in the direction of arrow A, and a full color image is formed on the sheet P. In the image forming apparatus with the indirect transfer system as shown in FIG. 13, toner images formed on the photosensitive elements 91Y, 91M, 91C, and 91K are sequentially transferred by superimposing, to an intermediate transfer belt 94 that rotates in the direction of arrow B. The toner images on the intermediate transfer belt 94 are collectively transferred to the sheet P, by a secondary transfer device 95. Note that a paper feed device 96 and a fixing device 97 are also shown in FIG. 12 and FIG. 13.
In the tandem type of image forming apparatus with the intermediate transfer belt as shown in FIG. 13, toner images of different colors formed on the photosensitive elements are superimposed on one another on the intermediate transfer belt to form a color image. Therefore, if positions on which the images are superimposed deviate from one another, color misalignment or a slight change in hue may occur in the color image. Thus, image quality degrades. Accordingly, the positional deviation (color misalignment) of the color toner images is a key problem.
Japanese Patent Application Laid Open No. H11-24507 (pages 3 to 4, FIG. 1) discloses a technology to correct unevenness in speed of a transfer belt in a color image forming apparatus using a conventional transfer belt.
In this technology, a color copying machine includes an intermediate transfer belt (or transfer belt) that is rotatably supported by five support rollers including one drive roller. Toner images of four colors of cyan, magenta, yellow, and black are sequentially transferred by superimposing to the circumferential surface of the intermediate transfer belt to form a full color image.
A scale with finely and accurately formed scale marks is provided on the internal surface of the intermediate transfer belt of the color copying machine. An optical detector (sensor) reads the scale to accurately detect the moving speed of the intermediate transfer belt. The moving speed detected is fed back by a feedback control system so that the intermediate transfer belt is made to move at an accurately controlled speed.
However, the scale may be worn out, damaged, or even dirty due to deposition of toner thereon, when the color copying machine is configured. Further, the scale with the scale marks formed along the belt is read by a sensor, the speed of the belt is detected based on information for the scale read, and the result of detection is fed back to controller so that the belt is made to move at an accurate speed. If the scale is worn out, damaged or dirty, the sensor may erroneously detect the scale mark(s) of the scale, thereby making it difficult to accurately control the belt speed.